Fig. 2.

Innate B cell-stimulating signals in the intestine. IECs release APRIL and trigger direct IgM-to-IgA1 CSR in LP IgM⁺ B cells and sequential IgA1-to-IgA2 CSR in LP IgA1⁺ B cells after sensing microorganisms through TLRs. iNOS⁺TNF⁺ DCs (TipDCs) and TLR5⁺ DCs cooperate with stromal cells and macrophages to further enhance local IgA CSR and production through a mechanism involving BAFF, APRIL, IL-6, IL-10, and TGF-β1. Similar signals promote the differentiation of IgA class-switched B cells into IgA-secreting plasma cells and facilitate plasma cell survival